1. Field of the Art
This invention relates to a wide angle lens for television cameras, and more particularly to a retrofocus type wide angle lens which is capable of making corrections to minimize variations in aberrations especially in field curvature and astigmatism which take place when focusing to an object at a close distance.
2. Description of the Prior Art
To meet a demand for a back focal length which is several times longer than the focal length, television camera lenses generally employ a retrofocus type lens system which is composed of a front group of lens elements with a negative refractive power and a rear group with a positive power. In focusing, it is the general practice with a retrofocus type lens of this sort to shift the position of the whole lens system forward or in a direction away from the image-forming plane. In this connection, there has also been known in the art to correct the field curvature, which varies in a marked degree especially in near distance focusing, by shifting the position of the negative front group rearward or toward the image-forming plane while sifting the whole lens system forward.
In the former case where the whole lens system is shifted forward at the time of focusing, it is necessary to correct aberrations on the basis of a reference object point located between infinite distance and close distances, in view of the lens characteristics of the asymmetric arrangement combining a negative front group with a positive rear group. Therefore, aberrations are suitably corrected at the reference object distance, but except the object point at the reference distance it is difficult to obtain images of good quality due to variations in aberrations especially in field curvature, which occur in a conspicuously magnified degree in infinite to near distance focusing. For example, if the whole lens system is shifted forward, various aberrations occur even in case of an optical system in one of the embodiments of the invention, as seen in the comparative examples shown in FIGS. 5, 9, 13 and 17 which will be discussed hereinlater.
On the other hand, in the latter case where the front group is shifted backward while shifting the whole lens system forward, it is possible to correct the field curvature which varies to a considerable degree in focusing to an object at the nearest distance. However, since this system is a wide angle lens with an extremely short focal length as compared with the back focal length, the negative refractive power of the front group becomes too strong to correct aberrations such as field curvature and astigmatism. That is to say, for correction of these aberrations, there have to be incorporated a larger number of lens elements of a larger diameter, which will result in equipments of heavy weight and awkward to handle in focusing operations, in addition to a problem that the correction of aberrations in near distance focusing is still unsatisfactory.